When Carey wrote about Biang!, the flagship of the Xi'an Famous Foods empire, in the middle of 2012, she described a meal at the restaurant as "generally just well-textured things dragged through awesome sauces. And I love it."
That's about as clear a distillation of Biang! as you need to know, and it explains this dish, Bao'Ji Mung Bean Jelly ($4), completely. Unlike other versions of liang fen that you'll find at Legend and Lao Cheng Du, Biang!'s isn't that spicy at all, and the wobbly, nearly flavorless mung bean jelly is cut into tiny noodle-like strips.
This means the sauce, a slightly sweet mix of chili and black vinegar with touches of scallion and garlic, flavors the jelly all the more. It's cooling with a light pickled vibe, and the absence of heat lets it function as a refreshing pause in between bites of fiery noodles and more assertive salads.